The present invention relates to a cylinder block structure for use in a multicylinder internal combustion engine.
One general approach to improve the performance of an internal combustion engine is to reduce the weight of the engine and increase the mechanical strength thereof to enable the engine to withstand high loads, so that the engine can produce a high power output for its weight and therefore it produces good fuel economy.
It is well known that the weight of an internal combustion engine can be reduced by casting its major structural component, the cylinder block, of a light alloy such as an aluminum alloy, as disclosed in U.S. Pat. No. 4,515,211 corresponding to Japanese Laid-Open patent publication No. 58(1983)-74851 published May 6, 1983. Since a light alloy is lower in mechanical strength than an iron alloy, it has been customary to form those portions that are subject to high loads, such as journal walls supporting the crankshaft, as thick walls or solid blocks for increased mechanical strength. Cylinder blocks of a light alloy are generally manufactured by the die-casting process. Since the molten metal solidifies at a high speed in the die-casting process, the thick walls or solid blocks formed of a large amount of molten metal tend to solidify at relatively widely different speeds at their different portions, resulting in casting defects such as voids or cavities produced therein. However, if the thick walls and solid blocks are eliminated from cylinder blocks, then the rigidity of the cylinder block normally would be reduced.
Another well known way of reinforcing cylinder blocks cast of a light alloy for producing greater rigidity and operation reliability has been to add reinforcing ribs to the cylinder blocks at portions where the mechanical strength is weaker than other portions, as shown in U.S. Pat. No. 3,977,385.